A steam turbine using operation steam to generate rotational power is formed in such a manner that a rotor shaft, an upper-half casing, and a lower-half casing are assembled together. Moreover, the steam turbine is provided with a diffuser (an enlarged flow path) formed to reduce an exhaust loss of the operation steam having been used for rotational power generation to exhaust the operation steam to the outside of the casing (see PTL 1). When the steam turbine is placed, a lower-half inner chamber and a lower-half outer chamber to which a lower-half flow guide is fixed are first placed on a foundation. Then, the rotor shaft to which a plurality of moving blades are fixed is placed rotatably about an axis. Subsequently, an upper-half inner chamber and an upper-half outer chamber to which an upper-half flow guide is fixed are fixed to the lower-half outer chamber.